Miniature inductors and transformers have recently been developed using very thin wall plastic tubes, in the order of 0.001 to 0.010 inches in thickness. Such tubes are made of extremely slippery plastic material having a low coefficient of friction. Such materials include, for example, various polyesters, polyamides, polyethylene terephthalate, fluorinated hydrocarbons and others plastics sold under such trademarks, for example, as Mylar, Teflon, Aramid and Kapton. Such plastics are used because their physical and electrical characteristics are ideal for use as thin walled dielectric tubes. Thin wall tubes permit the tuning element to be positioned close to the coil winding for improved inductance characteristics. Many of these plastics have such low coefficients of friction that it has been impossible to provide satisfactory means for securing a tuning element in the tube at variable selected locations. Normally the tuning element is a threaded element which screws into and out of the tube for tuning purposes. Unfortunately, no satisfactory means have been developed to provide internal threads on the inner surface of tubes formed of such plastics for the purpose of adjustably securing the tuning element within it. Some attempts have been made to crimp, dimple or form the thin wall tube with appropriate surface projections on the inner surface. This is difficult because these plastics have extreme memory properties or high coefficients of elasticity. Consequently it has not been possible to permanently form satisfactory dimples, crimps or other shapes on the inner surface of these thin wall tubes. Attempts have also been made to provide suitable threading surfaces on the inside of these tubes by lining the tubes with paper or plastic. Using such liners increases the wall thickness of the tube, thereby decreasing tuning efficiency. Other attempts have been made to provide a tube with a self threading screw surface. However, the plastic material is so hard that it cannot readily be scored by self threading tuning screws. Thus miniature inductors and transformers using ultra thin wall tubes have not been made, heretofore, on commercial scales with any degree of success.